A novel split-cylinder airlift reactor for fed-batch cultures

Abstract

Conventional airlift reactors are not adequate to carry out variable volume processes since it is not possible to achieve a proper liquid circulation in these reactors until the liquid height is higher than that of the downcomer. To carry out processes of variable volume, the use of a split-cylinder airlift reactor is proposed, in the interior of which two multi-perforated vertical baf¯es are installed in order to provide several points of communication between the reactor riser and downcomer. This improves the liquid circulation and mixing at any liquid volume.In fed-batch cultures, it is important to know how liquid height affects the hydrodynamic characteristics and the volumetric oxygen transfer coef®cient since this impacts on the kinetic behavior of any fermentation. Thus, in the present work, the effect of the liquid height on the mixing time, the overall gas hold-up, and the volumetric oxygen transfer coef®cient of the proposed airlift reactor were determined.The mixing time was increased and the volumetric oxygen transfer coef®cient decreased due to the increase of the liquid height in the reactor in all the super®cial gas velocities tested, which corresponded to a pseudohomogeneous¯ow regime. The experimental values of the mixing time and the mass-transfer coef®cient were properly described through correlations in which the U GR /H L ratio was used as the independent variable. Thus, this variable might be used to describe the hydrodynamic behavior and the oxygen transfer coef®cient of airlift reactors when such reactors are used in processes where the liquid volume changes with time.However, these correlations are useful for the particular device and for the particular operating conditions at which they were obtained. These empirical correlations are useful to understand some factors that in¯uence the mixing time and volumetric oxygen transfer coef®cient, but such correlations do not have a suf®cient predictive potential for a satisfactory reactor design.The overall gas hold-up values were not signi®cantly affected when the liquid height was lower than the downcomer height. However, the values decreased abruptly when the reactor was operated with liquid heights over the downcomer height, especially at high super®cial gas velocities.